This paper reviews the temporal dispersive effects suffered by electromagnetic pulses during transmission or reflection by the ionosphere. A qualitative description of intrinsic plasma, geometrical (refractive and diffractive), and geomagnetic dispersion is presented. Also, tables that summarize previous research studies are given to indicate the variety of pulse signal formats and ionospheric conditions that have been considered.
The time and frequency selective behavior of a fading HF ionospheric reflection channel is considered in this paper. Both deterministic and random spatial and temporal variations of the ionosphere and the troposphere are included in determining the multipath delay and Doppler frequency spreading of a channel in which the signals are reflected off of a multilayered ionosphere. An example which illustrates the diffuseness of the multipath and Doppler spectra under typical propagation conditions is given.
The group velocity of a wave which propagates over an impedance plane (ground wave) is frequency dependent. Therefore when a narrow pulse (wideband signal) propagates over such a surface, it exhibits delay dispersion. Previous efforts to determine the delay dispersion of wideband ground wave signals have been limited to propagation over a smooth spherical earth. The analysis presented in this paper includes the effects of surface roughness and is applicable to the case of a rough ocean surface. The results are presented as a function of distance and sea state. It is shown that at short distances surface roughness tends to increase the amount of delay dispersion. Beyond a certain distance, delay dispersion depends on roughness: a small amount of roughness surprisingly causes a reduction of the delay dispersion, a large amount of roughness causes increased delay dispersion as expected.
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